1. Field of the Invention
The present invention relates to a hold-type display device such as a liquid crystal display device. More specifically, the present invention relates to suppression of motion blur generated in such display device.
2. Description of the Related Art
FIG. 19 is a time chart showing changes in relative luminance of signals inputted to a hold-type display device such as a liquid crystal display device of a related technique with respect to the time base. FIG. 19(a1) and FIG. 19(a2) show chronological changes of the relative luminance in a case of normal drive, FIG. 19(b1) and FIG. 19(b2) show a case of black insertion drive depicted in Japanese Unexamined Patent Publication 2001-042282 (Patent Document 1), and FIG. 19(c1) and FIG. 19(c2) show a case of time-division drive depicted in Japanese Unexamined Patent Publication 2002-023707 (Patent Document 2). FIG. 19(a1) shows the case of the normal drive where the gradation of an input signal is small, while FIG. 19(a2) shows the case of the normal drive where the gradation of the input signal is large. Similarly, FIG. 19(b1) and FIG. 19(b2) as well as FIG. 19(c1) and FIG. 19(c2) respectively show the cases where the gradation of the input signal is small, and the cases where the gradation is large.
In the cases of the normal drive shown in FIG. 19(a1) and FIG. 19(a2), the relative luminance does not change with respect to the time base. The response time for the change of the input signal in the hold-type display device is longer than the response time of a cathode-ray tube display device, a plasma display device, and the like. In particular, due to the viscosity of the liquid crystal and the layer thickness, the change in the alignment of the liquid crystal in the liquid crystal display device is delayed compared to the change in the input signals due to its operational principle. Therefore, especially in a moving picture where the change in the input signals is large, a displayed screen may have an afterimage. This phenomenon is called a moving picture tailing (referred simply as “tailing”), a motion blur, or the like.
The black insertion drive of Patent Document 1 shown in FIG. 19(b1) and FIG. 19(b2) is proposed to suppress the motion blur. With this driving mode, the time base is divided into video sub-frames 301 and black sub-frames 302 in a prescribed proportion, and an inputted signal is displayed as it is in the video sub-frames 301, while a black screen with luminance of 0 is displayed in the black sub-frames 302. This action is the same for both of the case where the gradation of the input signals is small and the case where it is large.
This drive is a spurious impulse drive, so that it is possible to suppress generation of motion blur. However, luminance 303 (shown with alternate long and short dash lines of horizontal direction) recognized by human beings is the average value of the video sub-frames 301 and the black sub-frames 302, so that the luminance efficiency is reduced to half. Thus, it is necessary to increase the luminance of a backlight for preventing a decrease in the luminance, which results in inducing an increase in the cost and power consumption.
The time-division drive of Patent Document 2 shown in FIG. 19(c1) and FIG. 19(c2) is proposed to suppress the motion blur and to improve the luminance efficiency at the same time. With this driving mode, a video signal is double-speed converted to divide one frame to two sub-frames 304 and 305. In the case of FIG. 19(c1) where the gradation of the input signal is small, the gradation of one of the sub-frames, 305, is set to be close to the minimum value, and the gradation of the other sub-frame, 304, is changed.
In the case of FIG. 19(c2) where the gradation of the input signal is large, the gradation of one of the sub-frames, 305, is changed, and the gradation of the other sub-frame, 304, is set to be close to the maximum value. Luminance 306 (shown with alternate long and short dash lines of horizontal direction) recognized by human beings is the average value of the sub-frames 304 and 305. Thus, the gradation of the sub-frames is so set that the average luminance becomes equivalent to the gradation of the original input signal.
More specifically, it is a method which: provides an attenuation signal generating circuit which divides one frame into a plurality of sub-frames, and performs division with an attenuation variable which changes depending on the extent of the luminance of the input video signals; displays the luminance signal of before the division in a preceding sub-frame; and displays the luminance signal of after the division in a following sub-frame.
With this method, it is possible to suppress generation of the motion blur on the low-gradation side without deteriorating the luminance efficiency, by setting the attenuation variable in such a manner that the display of a following sub-frame becomes the maximum when the luminance of the input video signal is the maximum, and setting the attenuation variable in such a manner that the display of a following sub-frame becomes the minimum when the luminance of the input video signal is small.
Other than Patent Document 1 and Patent Document 2 described above, there are following technical documents which are related to suppression of motion blur generated in a hold-type display device. Domestic Re-publication of International Application WO 2003/032288 (Patent Document 3) discloses a technique which sets a period for displaying a black sub-frame in black insertion drive based on move amount of image signals. Japanese Unexamined Patent Publication 2007-133051 (Patent Document 4) discloses a technique which, in time-division drive, decreases the gradation of the sub-frame having the larger gradation to keep luminance difference between the sub-frames.
FIG. 20 shows graphs showing corresponding relations between input signals and relative luminance in each of the modes shown in FIG. 19. FIG. 20(a) shows a case of normal drive, FIG. 20(b) shows a case of black insertion drive, and FIG. 20(c) shows a case of time-division drive, respectively. In all the graphs, the lateral axis is the gradation of an input signal, and the vertical axis is the relative luminance with respect to a white screen. Naturally, there is no change in the relative luminance in the case of the normal drive shown in FIG. 20(a). In the case of the black insertion drive shown in FIG. 20(b), the relative luminance decreases in accordance with a proportion (1:1 in this case) of time periods where the video sub-frame 301 and the black sub-frame 302 are displayed, respectively.
In the case of the time-division drive shown in FIG. 20(c), when the gradation of the input signal is small, one of the sub-frames, 305, comes to have the relative luminance that is close to black display. Therefore, there is an effect of suppressing the generation of the motion blur as in the case of the black insertion drive. In addition, when the gradation of the input signal is large, the other sub-frame, 304, comes to have the luminance close to the maximum display. Therefore, there is also such an effect that the luminance efficiency is not deteriorated.
However, the effect of suppressing the generation of the motion blur is limited to the case where the gradation of the input signal is small. When the gradation of the input signal becomes larger, the effect of suppressing the motion blur becomes insignificant. Therefore, deformation of the moving picture, colored motion blur, and the like generated along with the motion blur are to generate continuously.
Further, while it becomes possible to suppress generation of the motion blur over the whole gradation ranges by having the attenuation variable as a constant and setting the luminance of the following sub-frame to be small, the effect of improving the luminance efficiency is decreased. That is, it is difficult with the above-described time-division drive to achieve both suppression of the motion blur and improvement of the luminance efficiency.
The technique disclosed in Patent Document 3 is a partially improved version of the black insertion drive disclosed in Patent Document 1. Since it is still the black insertion drive, the above-described shortcomings cannot be overcome with that technique. Further, since the technique disclosed in Patent Document 4 is to keep the luminance difference between the sub-frames by decreasing the gradation of the sub-frame having the larger gradation, deterioration in the luminance efficiency cannot be avoided, either. That is, none of Patent Documents 1-4 discloses a technique that is capable of achieving both the suppression of the motion blur and the improvement of the luminance efficiency.